Center array of objects in relation to another object
Press Space and write align objects into the search box that appears, then Enter



Then on the undo panel on the bottom left click on X and then Shift-click on Y



Finally to place exactly on top, enable snapping to vertex and move along the Z axis only (either by dragging the arrow or with G Z

The place where the mouse is when you hit G usually denotes the vertex you want to snap to the point the mouse is when you click to release. I say usually because depending on the camera angle blender might misunderstand your intentions.